Problem: In chemistry class, the girl to boy ratio is $5$ to $5$. If there are a total of $40$ students, how many boys are there?
Answer: A ratio of $5$ girls to $5$ boys means that a set of $10$ students will have $5$ girls and $5$ boys. A class of $40$ students has $4$ sets of $10$ students. Because we know that there are $5$ boys in each set of $10$ students, the class must have $4$ groups of $5$ boys each. There is a total of $20$ boys in chemistry class.